1. Field of the Invention
The present invention is concerned with providing security substrates, and documents made therefrom, such as banknotes, with features for visual inspection by members of the public. More specifically the invention relates to a novel security substrate containing at least two elongate security elements for the purposes of public and non-public verification.
2. Description of the Related Art
The inclusion of elongate elements, or security threads, into security paper is well known and has been widely described in the prior art. Such threads may be wholly or partially embedded into the paper. Partially embedded threads are commonly referred to as being windowed, as the thread surfaces at regular intervals on the paper surface like a series of windows. A number of methods for producing security papers with such windowed threads are known, one of which is described in EP-A-059056. Paper is still regularly produced by this method and sold commercially under the trade name Stardust®.
EP-A-059056 describes a method of manufacture of windowed thread paper on a cylinder mould papermaking machine. The technique involves embossing the cylinder mould cover and bringing an impermeable elongate security element into contact with the raised regions of an embossed mould cover, prior to the contact entry point into a vat of aqueous stock, referred to as a “Stardust track”. Where the impermeable security element makes intimate contact with the raised regions of the embossing, little or no fibre deposition can occur. After the paper is fully formed and couched from the cylinder mould cover, the water is extracted from the wet fibre mat and the paper is passed through a drying process. In the finished paper the contact points leave exposed regions of the security element which ultimately form the windows, which are visible in reflected light, on one side of the paper.
One problem which can arise in the production of windowed paper is where the threads are embedded in exactly the same position in every sheet. This means that the paper is extra thick in the region of the thread and problems arise in the paper finishing processes, especially during guillotining, as the stack of paper is markedly higher where the threads overlie each other. The problem is commonly solved by deliberately wandering the cross directional position of the thread, within a region typically 12 to 18 mm wide.
The use of windowed security threads has proved to be a highly effective security feature. However, as threads have developed and become more complex with the introduction of text, colour shifting features and holographic designs, there has been an increasing need to increase the width of the threads and thus the visual impact associated with the thread. This is particularly the case for holographic threads where the visual impact of the thread is very much dependent upon the area that is exposed and thus viewable. For threads bearing text, the wider the thread the bigger and, therefore more readable the text is. To this end, there has been a constant drive amongst security paper makers to produce security paper with wider threads.
The method described in EP-A-059056 has therefore been developed and enhanced to enable the embedding of wider threads into the paper substrate. EP-A-860298 describes one approach for embedding wide threads, that is threads having a width 2 mm or greater, into paper. A first paper web is manufactured according to the method described in EP-A-059056 and to this a second thinner paper web is applied, thus masking any fortuitous flaws on the reverse of the first paper web. Though effective, the method described in EP-A-860298 is not suitable for all types of paper machine.
Another alternative approach to the embedding of wider threads is described in patent specification WO-A-03095188. Here the shape of the bridges, which are formed between the windows, is modified to allow for improved water dispersion and to prevent the bridges splitting as the paper passes through the press section of the paper machine. This method is suitable for threads up to 6 mm wide, although the stated preferred width is 4 mm.
The thread width at which defects, such as poor window definition, bridge splitting and thread show through on the back side of the paper, become unacceptable is not only a function of the production method, but is also a function of the end use application. For example some users will require a higher quality paper than others, resulting in a narrower limitation to the thread width. Applications in which the finished document is only viewed from the front side are not limited by defects on the backside of the document, which is the side opposite to the windows in the case of windowed threads.
Furthermore it has been found that the production of paper with wide threads up to 6 mm wide, but more commonly between 2 and 4 mm wide, can limit the paper machine speed.